Tunable lasers based on color centers in alkali halides have been demonstrated over the spectral range from 0.8 to 4 .mu.m. [Mollenauer in "Tunable Lasers" ed Mollenauer et al, Springer-Verlag Berlin 1987, Vol. 59, p 227.]
In most alkali halides a pair of N-absorption bands, called the N.sub.1 and N.sub.2 bands, is observed on the long-wavelength side of the F.sub.2 band. The structure of the N-centers responsible for these absorptions has not been conclusively determined, although these centers have been studied for several decades. In previous work it has been suggested that the N-centers are neutral aggregates consisting of three or four F centers [Schnatterly et al, 1964, Phys. Rev., 135:A227; Compton et al, "Solid State Physics" ed Sertz et al, Academic Press. NY 1964, Vol. 16, pp 135-136; Nosenzo et al, 1970, Phys. Stat. Sol., 38:369; Hattori, 1962, J. Phys. Soc. Japan, 17:1454]. One model suggests that both bands originate from a trigonal F.sub.3 (trigonal F.sub.3 is also known as R) defect as a result of splitting of energy levels due to the Jahn-Teller effect [Silsbee, 1965, Phys. Rev., 138:A180; Rolfe et al, 1977, Phys. Rev., B15:3211]. Another model attributes the N.sub.1 band to the F.sub.4 -planar and the N.sub.2 band to the F.sub.4 -tetrahedral color center configurations [Pick, 1960, Z. Physik, 159:69]. Of these, in the case of the N.sub.2 band, the F.sub.3 model has received most experimental support [Silsbee, supra; Okamoto, 1961, Phys. Rev., 124:1090 (1961): Bron, 1962, Phys. Rev., 125:509; Schneider et al, 1965, Phys. Rev., 140, A1983; Schneider et al, 1966, J. Phys. Chem. Solids, 27:805; Schneider, 1966, Solid State Commun., 4:91; Schneider, 1986, J. Appl. Phys., 59:1086]. Much of the ambiguity is a result of evidence that, at least in KCl, other color center absorption bands overlap the N.sub.1 and N.sub.2 bands; Nosenzo et al, supra; Silsbee, supra; Rolfe et al, supra; Schneider et al, 1965, supra; Schneider et al, 1966, supra; Schneider, 1966, Solid State Commun., 4:91; Schneider, J. Appl. Phys., 59:1086.